Magnetic susceptibility is a measurement of the difference between the magnetic permeability of a material and the magnetic permeability of vacuum, typically expressed in parts per million (ppm) difference relative to vacuum. A material with a non-zero magnetic susceptibility, subjected to a uniform magnetic field in a vacuum, will distort the magnetic field by way of differences in the magnetic flux density inside the material versus outside it. This distortion causes localized changes in both the magnitude and direction of the applied magnetic field in the vicinity of the material. Many magnetically sensitive instruments can be sensitive to distortion of the associated magnetic fields, and can thus experience a performance degradation when exposed to magnetic field gradients. As an example, it can often be ideal for magnetic fields in a given instrument or enclosure to be approximately uniform.